At the present time, the tendency in telephone installations is to have many extensions on the same line. The electromechanical ringers or bells now commonly on telephone instruments consume relatively large amounts of power so that three or four instruments at the most can be connected to a standard telephone line and still have the ringers function. Even if greater amounts of power could be supplied for actuating the ringers, the standard telephone lines are not adequate to carry that power.
Present ringers or bells are also limited by their construction and principle of operation. As the ringer is electromechanical, a coil of wire is employed and for compatibility with the ringer signal, this coil is composed of many turns of fine wire. The coil, in turn, is mounted within a magnetic circuit comprised of several different magnetic or permeable elements. Moreover, it must be tuned to a particular ringing frequency by careful selection of the vibrating masses and spring forces. Thus, it must be a precision-manufactured instrument of subtle complexity and considerable expense. These principles of design and operation preclude the possibility of miniturization. Thus, a ringer of this size and expense has had a definite influence of the physical design of more streamlined and compact or more stylish telephones in the future as well as add to the retail price of such telephones. Also, to some people the usual bell has a harsh and irritating sound.
Others have developed or attempted to develop electronic ringer substitutes. The resulting hardware has had limited application in office phones and PBX systems. Low volume outputs, lack of volume control, size reliability and noise pick-up were all limiting factors. An overriding problem in the design of circuit to replace a telephone ringer is that a simple electronic tone emitter is undesirable due to spurious sound being generated when any electrical noise, i.e., voltage pulses other than ringing voltage, is present on the line to the ringer. These noise pulses can be generated from dialing, picking up the receiver, etc.